Precise in vivo impedance measurement of biological tissues allows for accurate evaluation of the physiological condition of the tissue and proper implementation of medical treatment. Elimination of errors caused by the measuring tool itself is critical to improved measurement accuracy and enhanced treatment effect.
Conventional impedance measurement methods for biological tissues suffer from the inability to differentiate the true impedance value from the errors induced by the measuring tool itself. It is difficult to eliminate such errors since they are inherent to the measurement method and are embedded in the measured signal from the measuring tool.
Referring to FIGS. 1 and 2, a measurement apparatus for measuring tissue impedance which demonstrates errors introduced into the measurement is shown. As seen in FIG. 1, two guidewires 10 and 11 are placed at or near a target tissue region 13 by traversing through body vessels. The impedance of the tissue region 13 (modeled as a capacitor and a resistor in parallel, Cu and Ru of FIG. 2, but could be any complex impedance) is of interest. The measurement may be made by a measuring device (not shown) connected to the proximal ends of each of the guidewires. Measurement error, however, may be created by measurement current flowing through the large capacitance to the body (the capacitance is modeled as capacitors C1 and C2, but is in fact distributed along the length of the guidewire). Capacitors C1 and C2 can be considered to be effectively in series with the patient's body, and will shunt away the current flowing through the tissue region 13 in parallel to C1 and C2, causing the measurement result to deviate from the true impedance values.
The measurement error can include, but is not limited to, common mode impedances, such as the shunt capacitance between the guidewires 10 and 11. Furthermore, the degree of measurement error may be influenced by certain conditions such as the electrical properties of the guidewires, especially the dielectric property of the shielding material of the two guidewires, the coating length and/or thickness of the shielding material of the two guidewires, the medium between the guidewires such, as tissue, fluid, and blood, or the driving signal.
Although methods have been developed to address certain measuring tool-induced errors, e.g., by making two impedance measurements to identify a true impedance value for the target tissue, limitations remain.